Illegal Immigration being used as wedge issue

In the Illinois 6th Congressional District race the Republican National Campaign Committee is attacking the Democratic candidate, Tammy Duckworth, an Iraqi War verteran, on illegal immigration. Rather than highlighting the real differences between the candidates on this issue, how we deal with the more than 10 million illegal immigrants already here, the above ads lie about Tammy Duckworth's stand on border security and "amnesty" for illegal immigrants.
According to the "bridget in the sixth" blog, the Duckworth campaign will be sending out mailers this week to deny the charges that she supports an "amnesty" and does not intend to secure our borders.

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There are real differences between Tammy Duckworth and her Republican opponent on immigration policy but the Republicans are not talking about those. Instead they lie about things almost all Americans (and almost all Congressional candidates, Democratic and Republican) agree on -- the need to secure our borders and stop illegal immigration. Why aren't the Republicans sending out mailers talking about the immigration issues that really divide us -- how do we deal with the more than 10 million illegal immigrants alreay here, many of whom have been here many years, have children who are citizens and have strong ties to the community? Unlike the Democrats the Republicans, insofar as they have a common approach, seem to favor threatening deportation to keep the millions of undocumented workers as permanent second-class citizens without the rights and protections other Americans enjoy. Why aren't the Republicans sending out mailers saying that? Could it be that their polls and focus groups tell them that approach does not appeal to a majority of voters?